A spark plug for use in an internal combustion engine is required to reduce its size or diameter for the purpose of, for example, improving the degree of freedom for design of the internal combustion engine. Specifically, as a result of reducing the diameter of the spark plug, a mounting hole for the spark plug can be reduced in diameter, whereby the degree of freedom can be improved for design of an intake port and an exhaust port. However, reducing the size or diameter of the spark plug is accompanied by a reduction in the diameter of an insulator, so that the mechanical strength of the insulator deteriorates. A deterioration in mechanical strength of the insulator may affect the performance of the spark plug.
For example, Patent Document 1 mentioned below discloses a spark plug in which a packing higher in hardness than a metallic shell is disposed between a diameter reducing portion (stepped portion) of an insulator at which the outside diameter of the insulator reduces, and a diameter reducing portion (stepped portion) of the metallic shell at which the inside diameter of the metallic shell reduces. In manufacture of the spark plug, when the spark plug is assembled through crimping, a portion of the packing is dug into the diameter reducing portion of the metallic shell, thereby providing a seal between the insulator and the metallic shell.